Power tongs are used in well drilling to rotate tubular sections that are being threaded together or unthreaded There exist numerous patents on power tongs, including the following recent U.S. Pat. Nos. 4,709,599 to Buck, 4,649,777 to Buck, 4,631,987 to Buck, 4,576,067 to Buck, 4,593,584 to Neves (equivalent to Canadian patent 1,235,111), U.S. Pat. Nos. 4,487,092 to Neves, 4,404,876 to Eckel, 4,350,062 to Farr et al, (equivalent to Canadian patent 1,125,737), U.S. Pat. Nos. 4,089,240 to Eckel, 4,084,453 to Eckel, and 2,879,680 to Beeman et al; and the following Canadian patents: 1,190,919 to Farr et al, 1,088,918 to Eckel 1,075,676 to Eckel, and 1,037,463 to Eckel.
All of these power tongs include a frame having arcuate front portions defining a throat, a cage plate assembly mounted on the frame for rotation about a central axis and a ring gear also mounted on the frame for rotation about the axis. The throat receives the tubular section to be rotated, and the cage plate and ring gear include die means for gripping the pipe. The ring gear and cage plate cooperate to rotate the pipe within the frame, which remains stationary. The design of all of these power tongs is such that the arcuate front portions of the frames are more rigid than the cage plate assembly and ring gear combination, and strain is transmitted directly from the ring gear and cage plate to the frame. In addition, the cage plate and ring gear are sufficiently flexible such that they may flex to a degree that teeth on the ring gear flex more than the backlash in the gear train that drives the ring gear. Finally, while several of the patents show offset cam surfaces, which help to ensure that the pipe is centrally located within the ring gear, the cam surfaces used are either circular or spiral.